Predatory Advantage’s ability has an “intervening ‘if’ clause.” That means (1) the ability won’t trigger at all unless the opponent whose turn it is didn’t cast a creature spell that turn, and (2) the ability will do nothing unless the opponent whose turn it is still hasn’t cast a creature spell by the time it resolves.

5/1/2009

Predatory Advantage checks whether a player cast a creature spell, not whether a creature entered the battlefield under that player’s control. If that player cast a creature spell but you countered it, for example, you won’t get a Lizard token that turn.

5/1/2009

In a Two-Headed Giant game, Predatory Advantage’s ability triggers twice at the end of the opposing team’s turn: once for each player. If neither of those players cast a creature spell that turn, you’ll get two Lizards. If one of those players cast a creature spell but the other didn’t, you’ll get one Lizard.